In a solid dispersion of a particulate functional compound as mentioned above, it is desirable that the functional compound is dispersed in a resin without making any aggregate and in its inherent finely particle size, in order to effectively exhibit its intended function.
In the past, a process for producing a solid dispersion of such a particulate functional compound was conducted by using a kneader which was equipped with a kneading shaft capable of heating and cooling, a vessel jacket, and a plunger for confining the kneaded product into the vessel under by slight pressure, and the process comprised the steps of:
(1) introducing into the kneader a finely particulate functional compound, a solid resin, an organic solvent for dissolving the resin, and additives as necessary, and then kneading the introduced mixture while volatilizing the organic solvent thereof by heat from the kneading shaft and the vessel jacket to produce a pitch-like highly viscous mass;
(2) roughly crushing the obtained highly viscous mass, subdividing the crushed matter into small quantities of between 10 and 30 kg, then passing the subdivided matter between two rolls to form a sheet, and folding the formed sheets to pass it again through the two rolls, and repeatedly carrying out the roll-kneading steps in between 30 and 100 times; and
(3) crushing the obtained roll kneaded product with a palletizing machine or a crusher to thereby obtain a final product.
Particularly, the above-described step (2) carried out by roll-kneading was important to finely disperse a functional compound into a resin, and thus was essential in the conventional process.
On the other hand, the above-described step (2) required a large amount of labor and energy, so that this step (2) has become a barrier to improved productivity. Accordingly, while the conventional method resulted in a high quality solid functional substance dispersion wherein the functional substance are dispersed in its inherent particle size, its production costs were high and its applications were limited to special fields.
In addition, in the conventional process, an organic solvent is introduced together with a functional compound and a solid resin, in order to soften the resin and to increase the affinity between the functional compound and the solid resin by wetting the functional compound therewith. However, in the process, the organic solvent, i.e. a hydrocarbon, is released into air, then the process was undesirable also in terms of atmospheric contamination. Further, since the organic solvent and the resin have a high affinity with each other, in some cases the organic solvent completely was not removed during the kneading step with heat and may remain in the kneaded product. When the obtained product is dissolved to use, the solubility of the product deteriorates being accompanied by volatilizing the organic solvent over time, which sometimes caused quality deterioration such as transparency, gloss and the like.
With respect to the various problems associated with this conventional method, the present inventors had found that impurity electrolyte prevents a functional compound from finely dispersing into a resin. Then, based on the discovery, the present inventors proposed a processes of finely dispersing a functional compound into a resin either by using a purified paste formed by purifying a water-containing paste of the functional compound with an ion-exchange method; or by preparing a water-containing paste from a dried pigment, then similarly preparing, from the water-containing paste, a purified paste with an ion-exchange method, subjecting the purified paste to a flashing process, namely a dispersing method in which a purified paste and a resin are kneaded and dewatered. (U.S. Pat. No. 6,136,907, and Japanese Patent No. 3069537)
These methods enabled a functional substance to be finely dispersed without any mechanical steps, and were revolutionary in terms of energy saving and environmental friendliness.
On the other hand, in this process, the purifying steps by ion-exchange caused relatively high equipment costs. Further, because the paste after purifying contained a relatively large amount of water content, the steps following the purification required fairly high costs and a fair amount of labor. Further, in the case of using a functional compound which contains a relatively large amount of impurity electrolyte, it was not easy to conduct the purification efficiently. Accordingly, there still are needs for Methods improved in terms of production costs and efficient production.
Moreover, if a resin having a high softening point was used, there was no alternative but to employ an organic solvent, so that the problem of the conventional methods of using an organic solvent has not been completely resolved.